The invention concerns a grazing angle microscope, in particular for use in connection with an infrared spectrometer, with a sample in a sample plane and optical means to, in a measuring configuration, either direct light under grazing incidence onto a small area of the sample surface and to guide the light reflected from this area to a detector and optical means to, in a viewing configuration, alternatively direct light at a steeper range of incident angles, preferably including orthogonal incidence, onto the same small area and to guide the light thereby reflected to viewing means.
Such a grazing angle microscope is known from U.S. Pat. No. 4,810,077 the entire disclosure of which is hereby incorporated by reference.
Many spectroscopic applications in industry and science require that sample surfaces be illuminated at grazing incidence. This means that the angle of incidence between the incident light beam and the normal to the sample plane is very large, i.e. larger than 60 degrees, usually larger than 80 degrees. The light reflected at grazing angles from the surface is guided to a detector of, for example, a Fourier transform infrared spectrometer (FTIR spectrometer).
In this context, if is often desirable to first visually view the area to be analyzed, to limit the area by means of a mask and then to measure the spectrum of this particular small area of the surface. The selectable area visible in the microscope should thereby coincide with the area investigated spectroscopically.
In the conventional grazing angle microscope, a sample surface is illuminated and viewed at grazing angles. By means of a mask, an enlarged image of a surface area with arbitrary geometrical contour can be selected and this area can be investigated spectroscopically. The microscope uses an objective for grazing incidence, wherein light, or more generally, electromagnetic radiation, first impinges on a central convex mirror on the optical microscope axis, is reflected by this mirror onto a concave, ring-shaped mirror and from this location is incident under grazing incidence on a small area of the sample surface. The reflected radiation emerging at grazing angles of reflection is first once more reflected from a ring-shaped concave mirror onto a second convex mirror and then reaches a detector for measurement. The small area investigated can be selected via a mask. Towards this end, either the incident or the reflected light is focused onto an adjustable mask at an intermediate focus to keep undesired light outside of the selected area of the surface. Along a second light path, coincident downstream of the mask with the one just described, separate illuminating light can be introduced with which the area on the surface can be selected, illuminated and viewed. The two convex and concave mirrors are each preferably manufactured as integral parts, so that both convex mirrors as well as both concave mirrors form a single component.
As already indicated above, in the present invention the expression "light" is not limited to the visible range but generally comprises electromagnetic radiation that can be focused by the optical components described. In particular, the infrared range is included. This is mainly relevant for the light that reaches the detector but also for the light that serves for visual viewing, which can be performed with a conventional ocular but also e.g. by attaching a video camera, which can also operate in the infrared range. Therefore, in principal, both the same as well as different light sources may be used for visual viewing and spectroscopy. In IR spectroscopy, it is common to switch light sources (and detectors) depending on the wavelength range, e.g. by shifting deflection mirrors. Clearly, this can also be done when switching to a separate viewing mode. Towards this end, it is particularly important that the selected surface area coincides in the viewing and measuring modes.
In order to make full use of the possibilities of measurements at grazing reflection it is necessary to employ polarized light. The band intensities of thin films on metal surfaces have strong p-polarization but extremely weak s-polarization.
There is therefore a need for a grazing angle microscope for spectroscopy of the above-mentioned kind, which permits use of polarized light.